1. Field of the Invention
The present invention relates to a method for producing vertical bipolar transistors having different voltage breakdown and high-frequency performance characteristics on a single die. The invention also relates to an integrated circuit comprising transistors of the above kind.
2. Description of the Related Art
Advanced silicon bipolar or BiCMOS processes are used to manufacture high-performance high-frequency circuits, commonly used for radio and analogue functions in mobile wireless devices such as base stations for cellular networks, mobile and wireless phones, wireless networking for computers etc. The bipolar transistors have good high-frequency and analogue performance, high gain, and are excellent for driving large currents. The CMOS technology is preferably used for logic, and thus the combined BiCMOS technology is an excellent choice for mixed-signal systems.
In high-frequency bipolar and BiCMOS processes, the bipolar transistor is implemented as a vertical device. The collector contact is placed at the side of the emitter/base structure. A buried collector, or subcollector, is used to achieve a low-ohmic path for the collector current from the active device via a highly doped vertical sinker to the collector contact. A common manner to form a buried collector is by doping the semiconductor substrate to high levels using preferably a patterned oxide mask, and then an epitaxial layer of silicon is grown on the substrate and the remaining parts of the transistor—base, emitter and collector contact—are formed on the top of the epitaxial layer.
The collector-to-base and collector-to-emitter breakdown voltages (BVcbo, BVces, BVceo) are important device parameters. The breakdown voltages depend on the thickness and doping of the lowly doped collector epitaxial layer. For the breakdown voltage BVceo, the current gain of the transistor is also involved, but the gain is set essentially by the emitter and base properties. As reach-through normally occurs prior to avalanche breakdown in the base-collector diode, the buried collector will affect the voltage breakdown. The distance between the buried collector and the base must be made short to insure good high-frequency performance and low collector resistance. However, the requested supply voltage will determine the minimum distance and thus affect the high-frequency performance. The trade-off between breakdown voltage and frequency performance in transistors is called the Johnson limit.
In a typical mixed-signal circuit design, not all the bipolar devices need to be optimized for best frequency performance. Some devices may instead need higher breakdown voltages, such as transistors used for switching current or used in interfaces to other circuits. However, simultaneous manufacturing of devices with high breakdown voltage and devices with good high-frequency performance on a single chip is not easily done without increased process complexity and increased costs associated therewith.
U.S. Patent Application No. 2003/0094673 by Dunn et al., discloses a solution to this problem by using two different impurity types, arsenic and antimony, and different concentrations for the buried collector. This approach requires additional process steps as well as additional photo masks.
U.S. Pat. No. 6,770,952 by Babcock et al., discloses the manufacturing of devices with different collector design on a single wafer. However, the solution utilizes thin silicon-on-insulator (SOI) technology, and can not be used in conventional BiCMOS technology on bulk wafers.